1. Technical Field
The technology described herein relates generally to wireless networking. More particularly, the technology relates to simultaneous communications between stations in a wireless network using one or more of Multi-User (MU) Multi-Input-Multi-Output (MIMO) and MU Orthogonal Frequency Division Multiple Access (OFDMA) technologies.
2. Description of the Related Art
Wireless LAN (WLAN) devices are currently being deployed in diverse environments. Some of these environments have large numbers of access points (APs) and non-AP stations in geographically limited areas. In addition, WLAN devices are increasingly required to support a variety of applications such as video, cloud access, and offloading. In particular, video traffic is expected to be the dominant type of traffic in many high efficiency WLAN deployments. With the real-time requirements of some of these applications, WLAN users demand improved performance in delivering their applications, including improved power consumption for battery-operated devices.
A WLAN is being standardized by the IEEE (Institute of Electrical and Electronics Engineers) Part 11 under the name of “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications.” A series of standards have been adopted as the WLAN evolved, including IEEE Std 802.11™-2012 (March 2012) (IEEE 802.11n). The IEEE Std 802.11 was subsequently amended by IEEE Std 802.11ae™-2012, IEEE Std 802.11aa™-2012, IEEE Std 802.11ad™-2012, and IEEE Std 802.11ac™-2013 (IEEE 802.11ac).
Recently, an amendment focused on providing a High Efficiency (HE) WLAN in high-density scenarios is being developed by the IEEE 802.11ax task group. The 802.11ax amendment focuses on improving metrics that reflect user experience, such as average per station throughput, the 5th percentile of per station throughput of a group of stations, and area throughput. Improvements may be made to support environments such as wireless corporate offices, outdoor hotspots, dense residential apartments, and stadiums.
An HE WLAN supports Down-Link (DL) and Up-Link (UL) Multi-User (MU) transmissions such as MU Orthogonal Frequency Division Multiple Access (MU OFDMA) transmissions and Multi-User Multi-Input-Multi-Output (MU MIMO) transmissions.
In an UL MU transmission, an Access Point (AP) may transmit a frame requiring an immediate response to a plurality of stations, such as a trigger frame or another type of frame. In response, the plurality of stations simultaneously transmit respective UL MU transmission frames to the AP.
Each of UL MU OFDMA frames transmitted by the stations may include a first portion transmitted across an entire bandwidth of a wireless channel and a second portion transmitted using only part of the bandwidth of the wireless channel. The respective first portions of the UL MU OFDMA frames are transmitted using a same range of frequencies (that is, using the same subchannels) as each other. In contrast, the respective second portions of the UL MU OFDMA frames are each transmitted using respective ranges of frequencies (that is, respective subchannels) allocated exclusively to each second portion.
A first WLAN device may transmit, over a channel, a frame that requires acknowledgement by an intended recipient of the frame. A second WLAN device that successfully receives the frame and that is the intended recipient of the frame may transmit an Acknowledgement (ACK) or a Block Acknowledgment (BA) frame to the first WLAN device to indicate that the frame was successfully received.
However, when the second WLAN device does not successfully receive the frame, the ACK or BA frame is not transmitted. Furthermore, even when the ACK or BA frame is transmitted, the first WLAN may not successfully receive the ACK or BA frame.
When the first WLAN device does not receive the ACK or BA frame, the first WLAN device may retransmit the frame after i) performing a Backoff procedure or a Point Coordination Function (PCF) IFS (PIFS) recovery, and 2) checking that the channel is idle. However, under some circumstances, performing the Backoff procedure or PIFS recovery and checking that the channel is idle may be unnecessary and may cause the capacity of the channel to be used inefficiently.
In a WLAN that supports MU MIMO or MU OFDMA, a single transmission by the first WLAN device may have a plurality of intended recipients and may require a plurality of independent ACK or BA frames in response. In such a WLAN, under some circumstances, performing the Backoff procedure or PIFS recovery and checking that the channel is idle when an ACK or BA frame is not received may be unnecessary and may cause the capacity of the channel to be used inefficiently.
Furthermore, when an AP solicits an UL MU frame requiring a plurality of immediate responses from a plurality of stations, under some circumstances, performing the Backoff procedure or PIFS recovery and checking that the channel is idle when an ACK or BA frame is not received may be unnecessary and may cause the capacity of the channel to be used inefficiently.
In a distributed wireless networks such as one operating according to an IEEE 802.11 standard, chances of multiple nearby stations transmitting simultaneously may be significantly reduced by utilizing a listen-before-talk protocol. In this protocol, when a station intends to transmit a frame, the station listens to the wireless medium first, and the station is allowed to transmit only when the wireless medium is not busy. In an IEEE 802.11 standard, the condition for the wireless medium being busy is extended to include virtual carrier sensing.
However, if the concept of random access is included into an IEEE 802.11 standard, because the transmitter of a trigger frame for random access does not know which station will participate in the random access, sometimes there will not be any responses to the trigger frame. This lack of response may not work properly with virtual carrier sensing, and thus may increase collision probability.
A WLAN that supports MU MIMO or MU OFDMA may include a process to allocate resources to random access communications, wherein one or more WLAN devices contend for use of the allocated resources without being individually scheduled to use the allocated resources. When a transmission by a WLAN device, such as an AP, explicitly or implicitly allocates resources of a following period of time to Random Access communication of other WLAN devices, performing a Backoff procedure or other error recovery procedure when no frames are received on the allocated resources may be unnecessary and may cause the capacity of the channel to be used inefficiently.
When a WLAN device allocates resources to random access transmissions of other WLAN devices, measures may need to be taken to prevent the channels, including the allocated resources, from being sensed as idle by WLAN devices not involved in the Random Access communications, that is, third party WLAN devices.
In distributed wireless networks such as a WLAN operated according to an IEEE 802.11 standard, a wireless medium is shared by many stations, and thus, packet transmission errors occur due to interference caused by multiple stations transmitting SU packets simultaneously. When a packet transmission error occurs, a retransmission process defined in the IEEE 802.11 standard may be performed.
However, when an UL MU simultaneous transmission is performed, a plurality of stations under an AP's control can simultaneously transmit to the AP in response to one or more trigger frames transmitted by the AP. For such UL MU transmissions, an IEEE 802.11n or 802.11ac scheme regarding the allocation of transmission bandwidth for response frames may not work properly, especially when the AP transmits more than one trigger frame simultaneously.